Historically, when photogrammetric techniques were employed to measure the amount of volume change in specimens being tested in a triaxial device, outside-of-the-cell cameras were utilized. However, light refraction at the 1) confining fluid-cell wall interface and 2) cell wall-atmosphere interface and the curvature of the cell wall have necessitated the use of models to account for the refraction and magnification effects. Furthermore, the cameras surrounding the testing apparatus have been expensive and have required an excessive amount of space to develop the required focal length and lighting conditions. Moreover, the optical elements of the camera equipment have not been addressed in detail, such as optical aberrations, inherent to the camera lenses.
Lenses are typically used to capture and focus light and may be used to increase the field of view. However, errors introduced by refraction of light through lenses, including spherical aberration, coma, field curvature, astigmatism, and barrel, pincushion, and complex distortions are prevalent to varying extents in lenses. Most camera lenses are therefore constructed of multiple lenses (lens array) that are stacked to correct for some aberrations. A careful balance between mitigating one type of aberration and augmenting another type has always existed; therefore it is never truly possible to capture an image that does not contain some type of aberration when a lens or lens array is utilized. Although most cameras use lens arrays, liquid lenses (with variable focus induced by an electrowetting process) have recently been developed for small applications to overcome aberrations encountered with lenses and to adjust the focal length without the need for mechanical servo action; liquid lenses are commonly utilized in many smart phone cameras. The aforementioned focus (inverse of power) of a given lens may be calculated utilizing the Lensmaker's equation (Equation 1) for thin lenses, first developed by English physicist Thomas Young.
                              1          f                =                              (                                                            n                  1                                                  n                  2                                            -              1                        )                    ⁢                      (                                          1                                  r                  1                                            -                              1                                  r                  2                                                      )                                              (                  Equation          ⁢                                          ⁢          1                )            
Where f is the focal length of the lens, n1 is the refractive index of the lens material, n2 is the refractive index of the surrounding medium, r1 is the radius of curvature of the front surface of the lens, and r2 is the radius of curvature of the back surface of the lens.
The pinhole aperture camera is the most basic type of camera and is often overlooked in favor of a lensed camera. However, despite, and perhaps because of its simplicity, the pinhole camera may provide: 1) images free of the optical distortions that are inherent to the use of lenses, 2) images with virtually infinite depth of field, 3) wide viewing angles, and 4) a foundation for understanding the basic concepts involved in the field of optics, specifically related to the use of cameras. The primary advantage of using a lens, as opposed to a simple pinhole, is that a lens can capture and focus more light, without requiring long exposure times, thereby increasing optical resolution (defined as the ability to resolve detail). When resolution is not the most critical objective of a camera application, a simple pinhole aperture camera may provide a viable alternative to a typical lensed camera. Pinholes have been used for centuries for purposes of viewing and tracing images onto drawings prior to utilizing photo-sensitive materials for photography purposes. Moreover, the basic concepts of pinhole optics were instrumental to the formulation of the theory of light. The theory was supported by the earliest written observations of multiple phenomena related to light, specifically diffraction, interference, and polarization of light, through pinholes.
The design of a pinhole aperture is relatively simple; however, certain considerations are necessary to optimize image quality. Unlike lensed cameras, pinhole cameras rely on diffraction, not refraction. The theory and equations for pinhole apertures were suggested by early researchers. Attempts have also been made to refine the relationship between the optical phenomena in more recent years. However, the theoretical limits should only be used as a guideline, because the optimal pinhole aperture diameter is often better determined experimentally. The optimal pinhole diameter is limited by resolution (larger diameters correspond with poorer resolution), by Fresnel (near-field) and Fraunhofer (far-field) diffraction limits, and by the ability to gather light (smaller diameters correspond with higher diffraction interference and allow less light to be collected). The optimal pinhole diameter for optical applications often relates to the “Airy disk” which is the bright, focused spot, central to a diffraction pattern through a perfectly circular aperture.
To be able to withstand high pressures, a camera is typically sealed in pressure-resistant or, more commonly, pressure-compensating housings. These housings are typically bulky, expensive, and do not allow for direct optical observation because light must first pass through a transparent thermoplastic barrier (i.e. acrylic plastic) before reaching the camera. To combat this, fluids such as silicone oil or mineral oil may be used in electronics applications where exposure to the fluid is unavoidable or desired. The direct contact between the electronics and fluid will not cause short-circuiting, due to the inert and non-ionic properties of the oil. Furthermore, even at high pressures, the silicone oil does not crush the components of the camera even though the components are directly subjected to the fluid. This direct immersion allows for pressure resistant design, without the need for a housing; thereby also allowing for direct optical observation.